Ischemia is a medical condition caused by an insufficient supply of blood to an organ, usually due to a blocked artery. Myocardial ischemia, for example, is an intermediate condition in coronary artery disease during which heart tissue fails to receive oxygen and other nutrients from the blood. Ischemia remains one of the most prevalent causes of morbidity and mortality in the developed world.
Ischemic episodes can be symptomatic or silent (i.e. without observable symptoms), and the presence or absence of symptoms appears to be independent of the severity of the ischemia. According to the American Heart Association, millions of Americans may have silent ischemia. If even minor forms of ischemia remain untreated, affected heart tissue can eventually die, placing the patient at a high risk of having a heart attack with little or no warning.
Myocardial ischemia is traditionally classified as “stable” or “unstable”, depending upon severity. Stable ischemia, the least alarming form of ischemia, is caused by a high demand for oxygen by the myocardium during exertion and is relieved by rest. Although stable ischemia is of medical concern, many unaware patients live with stable ischemia for months or years without serious medical repercussions. Unstable ischemia, which occurs when the patient is at rest or is not relieved by rest, is caused by a sudden decrease in blood supply to the heart and is generally considered to be a medical emergency. This type of ischemia is frequently caused by atheroschlerotic plaque rupture and clot formation inside the arteries. While unstable ischemia occasionally resolves spontaneously with no long-term sequelae, the condition frequently persists until myocardial infarction occurs.
Physicians consider various factors in further classifying the severity of stable and unstable episodes of ischemia. Even stable ischemia can be of great concern if it affects a relatively large section of the myocardium, for example, or if the episode persists for a relatively long period of time (e.g. on the order of thirty minutes or so), since such episodes can lead to myocardial cell stunning or death. Further, physicians often compare episodes of ischemia to prior episodes experienced by the patient to identify a typical episodes.
Although many techniques of diagnosing ischemia have existed for some time, these techniques have typically required the patient to consult a health care provider to be effective. Electrocardiograms (ECG) or electrograms (EGM), for example, have been shown to be effective in diagnosing ischemia by identifying abnormalities in the patient's cardiac activity. Variations in the ST segment of the patient's “PQRST” ECG waveform, for example, have been shown to indicate episodes of myocardial ischemia. U.S. Pat. No. 6,128,526 to Stadler et al. describes one type of ischemia detector that observes variation in the ST segment to identify an ischemic condition. Other ischemia detection techniques have relied upon measures of heart activity, patient workload and other factors.
Early detection of myocardial ischemia provides the opportunity for a wide range of effective therapies such as revascularization, neural stimulation, and drug delivery to reduce cardiac workload or to improve cardiac circulation. Because many episodes of myocardial ischemia occur without causing excessive pain or other noticeable warning signs, however, ischemia frequently goes undetected. Gauging the severity of ischemia has historically required that the patient be physically present at a health care facility to obtain diagnosis from an ECG, EGM or the like. As a result, ischemia frequently remains untreated until a major episode occurs and the patient arrives at an emergency room or trauma center. Accordingly, it is desirable to create a technique for identifying ischemia at an early stage, while the condition remains highly treatable and before a major episode occurs. In addition, it is desirable to create a device and/or technique that is capable of gauging the severity of ischemia so that the condition can, be appropriately treated. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.